Quantum emitters (including alkali atoms, trapped ions, solid state defects, and quantum dots) strongly coupled to photons have a wide range of technological applications in photonic computing, quantum information processing, quantum sensing and metrology, and quantum networks. The requirements for most of these applications are that photons interact with the emitter with high probability (the emitter has a large absorption cross section), and are collected with high probability. One major prerequisite to fulfil these requirements is that the emission be Fourier transform-limited, i.e. that the line width of the emission be determined only by the lifetime of the excited state. When the line width is broader than this transform-limited line width, additional de-phasing leads to a decrease in the absorption cross-section and a decrease in the effective collection efficiency. A broader line width can be due to coupling to phonons, spectral diffusion, or other non-radiative decay channels.
A quantum emitter with a transform-limited line width that is well localized to a single layer in a solid-state material can be used to construct devices that enable strong single-photon nonlinearities. By coupling a quantum emitter to a single mode waveguide, for example, one can realize a single photon transistor, in which the propagation of one photon is controlled by the internal state of the emitter, which is in turn manipulated by another photon. Such a device is a key building block for photonic computing platforms.
Furthermore, quantum emitters such as a nitrogen-vacancy (NV) center in diamond can be used as nanoscale magnetometers, and increasing the photon collection efficiency can improve the sensitivity drastically. An NV center with a transform-limited zero phonon line (ZPL) can be used to detect external spins with high sensitivity. For this application, it is also important that the NV be localized close to the diamond surface, since magnetic fields from external sources decay rapidly with distance.
Additionally, NV centers with transform-limited line widths can be employed in hybrid quantum systems, in which the NV center is coupled to another quantum system such as a superconducting circuit for microwave to photon conversion, optomechanical systems for photon-phonon conversion, and optoelectronic devices.
Multi-photon quantum entanglement from multiple photon emitters is considered to be a key ingredient for certain quantum processing applications. To achieve such entanglement requires the photons from different emitters to be quantum mechanically indistinguishable.
Multi-photon quantum entanglement from multiple gas state emitters, including emission from a single atom/ion in a trap, is known. This is achieved by generating photon emission from gas state emitters which is identical in terms of bandwidth, frequency, and polarization such that photons from different emitters are quantum mechanically indistinguishable. These identical photons can then be overlapped in a beamsplitter to achieve remote quantum entanglement.
The aforementioned approach is problematic for solid state emitters. This is because the energies of optical transitions in solid state systems vary due to variations in the electronic environment and strain within solid state crystal systems. Differences in emission characteristics of solid state emitters may be caused by impurities, intrinsic crystal defects such as dislocations, extrinsic defects such as those resulting from processing damage, and/or other extrinsic effects such as Stark tuning by electric field. As such, photons emitted from two different solid state emitters vary in terms of bandwidth, frequency, and polarization and are quantum mechanically distinguishable. Accordingly, such photons do not undergo quantum entanglement when overlapped on a beamsplitter or comparable arrangement.
One solution to the aforementioned problem is to reduce the resolution of a detector arrangement used to detect photons from photon emitters to an extent that the photons from different sources are indistinguishable to the detectors. For example, by using detectors with a high time resolution this in turn leads to a low frequency resolution, which can render the photons indistinguishable. However, the higher timing resolution requires that the count rates in each bin be higher than noise, such as dark counts, and single photon emission from defects in solid state materials can be very weak. For example, the photon emissive nitrogen-vacancy defect (NV−) in diamond material, which is a leading candidate for solid state quantum processing applications, exhibits a broad spectral emission associated with a Debye-Waller factor of the order of 0.05, even at low temperature. Emission of single photons in the Zero-Phonon Line (ZPL) is then typically of the order of a hundred thousand of photons per second. Due to poor collection efficiency only approximately 0.1-1% of this emission is typically detected resulting in low count rates. Such count rates are insufficient for the realization of advanced quantum information processing protocols within reasonable data acquisition times based on photon interference using high time resolution. (i.e. low frequency resolution) detectors.
In fact, the aforementioned issues are so problematic that multi-photon quantum interference from multiple solid state quantum registers had not been demonstrated in practice until 2011 (Phys. Rev. Lett. 108, 043604 (2012). In this regard, it should be noted that a solid state quantum register can comprise both nuclear and electron spins coupled together. An electron spin can function as a control qubit with optical spin state detection and fast high fidelity coherent manipulation. A nuclear spin can function as a memory qubit which has weak interaction with the surrounding environment. Together, an electron spin and a nuclear spin coupled can form a quantum register. An example of such a quantum register is a nitrogen-vacancy defect in diamond material which has resolvable electron spin states that are optically addressable and coupled to nuclear spin states of the nitrogen nucleus and/or 13C nuclei in the surrounding diamond lattice. It should be noted that a quantum register of this kind differs from systems which only comprise single spin emitters, decoupled spin states, or emitters which do not comprise spin states which can be resolved to function as a quantum register.
In relation to the above, it is noted that multi-photon interference has been observed from more simple solid state systems such as quantum dots, single molecules adsorbed onto a surface, and F-dopants in ZnSe [see, for example, R. Lettow et al., Physical Review Letters 104 (2010); Patel et al., Nature Photonics 2010, DOI: 10.1038/NPHOTON.2010.161; Sanaka et al., PRL 103, 053601 (2009); and Flagg et al., Phys. Rev. Lett. 104, 137401 (2010)]. However, multi-photon quantum interference from multiple spin resolved solid state quantum registers has not been demonstrated in practice to date due to the previously described problems. Quantum dots in III-V semiconductors relate to electronic transitions in a dense nuclear spin bath and thus are unsuitable for quantum information processing applications requiring a nuclear spin memory qubit. Single molecules adsorbed onto a surface do not exhibit spin resolved emission suitable for information processing. Furthermore, surface crystallized single molecule systems are inherently fragile systems which may be unsuitable for commercial device applications. F-dopants in ZnSe have not been demonstrated to comprise spin resolved solid state quantum registers in which resolvable electron spin states are coupled to one or more nuclear spins.
In contrast to the above, in 2011 multi-photon quantum interference from multiple spin resolved solid state quantum registers was demonstrated for NV− spin defects in CVD synthetic diamond material. This was achieved by providing a combination of features including:                (i) Synthesis of very high purity, low strain CVD synthetic diamond material comprising NV− spin defects in a relatively uniform electronic and strain environment;        (ii) Selection of two NV− spin defects with nearly identical frequency within the high purity, low strain CVD synthetic diamond material;        (iii) Fabrication of solid immersion lenses in the high purity, low strain CVD synthetic diamond material over each of the selected NV− spin defects to increase optical out-coupling of photons emitted by the NV− spin defects;        (iv) Stark tuning of the NV− defects to reduce the difference in frequency between the two NV− spin defects;        (v) Filtering of the photons emitted by the two selected NV− defects using a dichroic mirror configured to separate a zero phonon NV− line emission from a phonon side band emission;        (vi) Further filtering of the emitted photons using a polarizing beam splitter;        (vii) Overlapping the filtered photons from each NV− spin defect on a fibre beam splitter; and        (viii) Detecting the tuned and filtered photon emission using a detector arrangement which is configured to resolve sufficiently small differences in photon detection times that the tuned and filtered photon emission from the NV− spin defects is quantum mechanically indistinguishable resulting in quantum interference between indistinguishable photon emission from the two NV− spin defects.        
The aforementioned approach has proved successful in demonstrating an approach to achieve multi-photon interference from solid-state emitters which may be used in quantum processing applications. However, the tuned and filtered photon emission is still relatively weak with a relatively low proportion of the emitted photons being detected resulting in low photon count rates and relatively long data acquisition times. For commercial devices it would be desirable to further increase the photon count rates and reduce data acquisition times.
Accordingly, there is still a need to provide a device which is capable of providing multi-photon quantum interference from multiple solid state quantum registers at faster data acquisition times.
In this regard, the inhomogeneous broadening and spectral variation relative to the natural emission line width of each solid state photon emitter will limit the observability of quantum interference as emitted photons will be spread over a range of frequencies such that a significant proportion of photons will be quantum mechanically distinguishable. Tuning and filtering the emission can be used in combination with a suitably configured detector to observe quantum interference as previously described but a significant proportion of photons are discarded in such an approach leading to relatively low photon count rates and relatively long data acquisition times. As such, it would be desirable to provide a synthetic diamond material which comprises spin defects having a narrower emission line width in order to reduce or eliminate the need for tuning and filtering of the emission to achieve multi-photon interference at higher photon count rates and reduced data acquisition times.
Fluorescence from the NV can be separated into two components with a standard branching ratio between them: the zero phonon line (ZPL) and the phonon side band (PSB). Typical branching ratios, as measured, are 1:20 or 1:30. The PSB is broad, spanning more than 100 nm, and results from vibronic transitions that cannot be frozen out (e.g., their intensity is determined by Franck-Condon factors between the excited state and various phonon excited, electron ground states, and the Debye-Waller factor is 0.03-0.05). The theoretical minimum line width of a solid state emitter is known as the (Fourier) transform-limited line width and is determined by the excited state lifetime of the emitter. For NV−, this is phonon broadened at room temperature to about 1 THz (Phys. Rev. Lett., vol. 103, issue 25, pp 256404, 2009) but below approximately 10 K, the line width can in principle be transform-limited. The excited state lifetime of the NV center has been measured to be 12 ns (τ), which is what limits the natural line width to δυ=(1/2πτ)=13 MHz in an ideal material. In almost all diamond materials (natural, CVD synthetic, high pressure high temperature (HPHT) synthetic, and nanodiamond) the line width is significantly larger than this fundamental limit and each defect shows variation. This is due to the fact that each defect has a different local electronic and strain environment. For example, it has been noted that even though the multi-photon interference configuration previously described utilized a CVD synthetic diamond material with very high purity and low strain to provide NV− spin defects in a relatively uniform electronic and strain environment, the emission line width for each NV− spin defect was still significantly higher than the theoretical limit of 13 MHz. Individual photoluminescent excitation spectra (single-scans), recorded in the absence of green light, showed a zero phonon line width of 36 MHz and 38 MHz for the two selected NV− spin defects with a 532 nm repump pulse between scans leading to an overall total distribution of frequencies with a line width of a few hundred MHz. Spectral diffusion was also observed in photoluminescent excitation spectra recorded with simultaneous green excitation and in the zero phonon line emission spectrum under 532 nm excitation. Emission line broadening over time is known in the art as inhomogeneous line broadening and the overall total spectral line width measured over time is known as the inhomogeneous line width. This contrasts with a line width measured at a specific instance in time known as the single scan line width. However, despite the observed inhomogeneous broadening exceeding the radiative line width by an order of magnitude, two photon interference effects were still detected due to the application of suitable tuning and filtering and a detector configured such that the zero phonon emission line width did not exceed the inverse time resolution of the photon detectors.
The table below gives some examples of zero phonon line widths measured in different types of diamond materials.
TypeSampleNV− zero phonon linewidthNV− spin defects in“Type IIa” crystal~50 MHznatural diamondmaterialsNaturally occurringA type IIa nanocrystal16 MHz (single scan,NV− spin defects in(Shen, PRB 2008)not including spectralsynthetic diamonddiffusion)materialsElectronic grade CVD~30 MHz to ~500 MHz(single scan, not includingspectral diffusion)Nanobeams in type IIa~10 s of GHzNV− spin defectsImplantation, bulk~100 MHz to few GHzintroduced into(single scan, not includingelectronic gradespectral diffusion)CVD diamondElectron irradiation,~30 MHz to 100 MHzbulk(single scan, notincluding spectraldiffusion)
Natural diamond materials have a large variety of NV− zero phonon line widths reflecting the large variety of diamond crystal structures found in nature. While every diamond crystal is unique, high purity type IIa natural diamonds can have a zero phonon line width of the order to 50 MHz. In one famous natural diamond from the Urals a stable zero phonon line width of less than 20 MHz has been observed. For example, P. Tamarat et al. [Phys. Rev. Lett. 97, 083002 (2006)] have studied this unique natural diamond and reported an NV− zero phonon line which is at the transform limited line width of 13 MHz and which is stable over many seconds and excitation cycles. However, the reasons why NV− spin defects exhibit such narrow and stable emission characteristics in this single natural diamond sample are not currently well understood. Furthermore, such narrow, stable zero phonon line widths have not been observed to date in any synthetic diamond materials.
Synthetic diamond materials also have a large variety of NV− zero phonon line widths reflecting the large variety of synthesis techniques and recipes for targeting different diamond materials having properties optimized for particular applications. Synthetic diamond materials can be distinguished from natural diamond materials using a range of spectroscopic techniques known in the art because synthetic materials have a different composition of extended defects (e.g. dislocations) and impurity defects when compared with natural diamond materials. Synthetic diamond materials are advantageous over natural materials for technical applications in that the use of a specific synthesis technique and recipe can lead to a reproducible product material.
Despite the above, at the date of writing this specification the present inventors are not aware of any disclosure of a route to achieving synthetic diamond material with a stable inhomogeneous spin defect zero phonon line width of less than 100 MHz. Where line width values below 100 MHz have been reported, these are measured over a single scan and do not include spectral diffusion.
An NV− zero phonon line width of 16 MHz has been reported by Shen et al. [Phys. Rev. B 77, 033201 (2008)] in a synthetic type IIa nanodiamond. However, this line width was measured from an individual photoluminescent excitation spectrum and is subject to spectral diffusion leading to line broadening over time. That is, the emission line frequency moves over time such that while at a single instance the line width may be narrow, when the line width is measured over a number of spectral scans summed together the overall distribution of frequencies results in an inhomogeneous line width greater than 100 MHz.
V. M. Acosta et al. [Phys. Rev. Lett. 108, 206401 (2012)] have recently reported the application of a dynamic feedback Stark tuning technique for reducing spectral diffusion of NV− emission frequency over time leading to spectral broadening. An average single scan line width of 140 MHz is reported for an NV− defect located in a 100 nm thick CVD synthetic diamond film deposited on a high purity 100-oriented diamond substrate. A much narrower NV− line width of 60 MHz is reported for a natural type IIa diamond sample. It is reported that spectral diffusion of a zero phonon emission line in the natural sample was supressed to 16 MHz standard deviation by application of the dynamic feedback Stark tuning technique. However, the 16 MHz figure represents the spectral drift from the starting line position and not the single scan line width which was unchanged by the dynamic feedback Stark tuning technique and remained at 60 MHz. In any case, the 60 MHz value is for the natural diamond sample rather than the synthetic diamond sample.
Other background references include the following:    Faraon et al. PRL 109, 033604 (2012) [http://prl.aps.org/pdf/PRL/v109/i3/e033604] discloses NV spin defects in a type IIa photonic device with a 4 GHz line width;    Kai-Mei C. Fu, PRL 103, 256404 (2009) [http://prl.aps.org/pdf/PRL/v103/i25/e256404] discloses NV defects in electronic grade material with an ˜30 MHz single scan line width; and    Bernien PRL 108, 043604 (2012) [http://prl.aps.org/pdf/PRL/v108/i4/e043604] discloses NV spin defects in electronic grade material with a 263 MHz inhomogeneous line width and 36 MHz single scan line width.
It should be noted that in the previous discussion references to the zero phonon line width of a spin defect have been in relation to the intrinsic line width of the emission with no photonic cavity broadening. In certain quantum or optical applications it is advantageous to couple one or more spin defects to an optical cavity or photonic cavity structure. Such a photonic cavity can be used to increase the number of photons emitted in the zero phonon line and also increases the width of the zero phonon line via cavity broadening, as a function of the Q-factor of the cavity. As such, in quantum device structures which utilize a photonic cavity the observed zero phonon line width will be larger than the intrinsic line width of the spin defect if no photonic cavity was present. It should be noted that even when a photonic cavity structure is provided it is still advantageous to provide a spin defect which has an intrinsically narrow zero phonon line width emission. The term intrinsic inhomogeneous zero phonon line width will be used herein to relate to the intrinsic inhomogeneous zero phonon line width of the spin defect with no photonic cavity broadening and with no Stark tuning to counteract spectral drift. If a photonic cavity is present, then the effect of cavity broadening can be subtracted from the observed zero phonon line width to determine the intrinsic inhomogeneous zero phonon line width of the spin defect. In order to extract the intrinsic line width of an NV center that is radiatively broadened by the presence of a cavity, time-dependent fluorescence measurements can be used to extract the lifetime of the NV center. This line width, measured by resonant absorption/fluorescence, is simply the inverse of the measured lifetime δυ=(1/2πτ) which will be a combination of the cavity broadening and the intrinsic line width. The maximum amount that the line width can decrease is given by the Purcell factor, assuming optimal NV alignment within the cavity. The Purcell factor can be calculated from the cavity Q and the mode volume. The cavity Q and resonance position can be characterized independently by a variety of techniques, such as transmission, tapered fiber coupling, and cross-polarized scattering. The mode volume is harder to characterize externally (typically being calculated or simulated) but can be done using near-field scanning techniques [Okamato et al, Appl. Phys. Lett. 82, 1676 (2003) and McDaniel et al, Phys. Rev. B 55, 10878-10882 (1997)]. Another approach to measure the actual contribution of the cavity to the NV line width is to tune the NV center and cavity out of resonance with one another. Typical achievable NV detunings by the Stark effect and micro-fabricated electrodes are of the order of 100 GHz [Phys. Rev. Lett., vol. 97, issue 8, pp 083002, 2006 and Phys. Rev. Lett., vol. 107, issue 26, pp. 266403, 2011]. This is feasible for cavities with Q>5000. It is also possible to tune the cavity by condensation of noble gases, deposition of other dielectrics, and etching. When the cavity and NV are detuned, the lifetime of the NV center should increase to its natural lifetime, and its line width should reflect the intrinsic line width of the NV without the cavity.
In addition to the above, it should also be noted that the method of measuring the zero phonon line width can affect the observed zero phonon line width. For example, measuring the zero phonon line width at temperatures significantly above the Debye Temperature leads to temperature induced phonon broadening. Furthermore, exciting the spin defect at high laser powers can lead to power broadening of the zero phonon line emission. Accordingly, in addition to the above, the term intrinsic inhomogeneous zero phonon line width will be used herein to relate to the inhomogeneous zero phonon line width which is measured at a sample temperature low enough to inhibit phonon broadening, and at a low enough excitation power to avoid power broadening.
It is an aim of the present invention to provide a synthetic diamond material and a method of synthesising such material which comprises solid state spin defect photon emitters with an intrinsic inhomogeneous zero phonon line width approaching that of the transform limited value and which is stable with minimal spectral diffusion. The intrinsic inhomogeneous zero phonon line width is measured and calculated to include intrinsic line broadening effects such as intrinsic spectral drift but to exclude extrinsic effects such as temperature induced phonon broadening, excitation induced power broadening, photonic cavity broadening, and Stark tuned narrowing of the inhomogeneous zero phonon line width of solid state spin defects. Such narrow line width, stable solid state photon emitters constitute a major step forward towards solid state quantum computing and are also useful in other quantum optics, quantum sensing, and quantum processing applications.